x11: emit ::enter/leave-monitor

x11: emit ::enter/leave-monitor

For the X11 backend, keep a list of monitors for which the surface
intersects the monitor area.

Whenever the X11 surface is configured, check against the list of
monitors to determine whether it enters a new monitor or if it left a
monitor, to emit the corresponding ::enter/leave-monitor signals just
like a Wayland compositor would.

As monitors can be added, removed or reconfigured at any time, redo
those checks whenever any of these events occur.